What are the 3rd and 4th tier schools?

My main problem was where to draw the line. I figured that with my 31 ACT, 4.0 GPA but with only 3 AP classes, weak ECs, I shouldn’t aim too high, so I was wondering where the third tier schools are at. I have not idea what to look for. Do acceptance rates, average GPAs and ACTs, research expidentures, # of students, students to teacher ratios determine this?